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Larvikite is a variety of monzonite, notable for the presence of handsome, thumbnail-sized crystals of feldspar. These feldspars are known as ternary because they contain significant components of all three end-member feldspars. The feldspar has partly unmixed on the micro-scale to form a perthite, and the presence of the alternating alkali feldspar and plagioclase layers give its characteristic silver blue sheen (Schiller effect) on polished surfaces. Olivine can be present along with apatite, and locally quartz. Usually titanium rich, with titanaugite and/or titanomagnetite present.
Intrusions of Larvikite form part of the suite of igneous rocks that were emplaced during the Permian period, associated with the formation of the Oslo rift. The crystallisation of a ternary feldspar indicates that this rock began to crystallise under lower crustal conditions.
This rock is very popular in the High Street in the UK, for its high polish and the spectacular reflections from its feldspars and is frequently found fronting the facades of banks. It is sometimes known informally as Blue Granite, although this is not an accurate description.
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